Conventional mass spectrometry primarily focuses on measuring the concentrations of isotopic species including only one rare isotope. These mass spectrometric techniques generally determine the overall concentration of an isotope, irrespective of its location in the molecule (i.e., the atomic site or sites of isotopic substitution) or the proportions of multiple isotopic substitutions within the same molecule. Consequently, conventional mass spectrometry fails to distinguish among different isotopologues of the same molecule and thus disregards a large amount of useful information that can be determined from a complete analysis of all the different isotopologues present in a sample. However, determining the isotopic composition of a molecule including more than one rare isotope can provide useful information, such as the geographic origin of the molecule, temperature of origin of the molecule (or a sample including the molecule) or the identity of a parent molecule from which the molecule was derived.
The shortcomings of conventional mass spectrometry are particularly noteworthy for organic compounds, which may have large numbers of isotopologues. For example, methane (CH4) has 57 distinct isotopic versions including various non-equivalent combinations of 12C, 13C, 14C, hydrogen, deuterium, and tritium. The number of isotopologues of low-symmetry molecular structures grows approximately geometrically with the number of atomic positions, meaning alkanes, lipids, sugars and other complex hydrocarbons containing several or more carbon atoms typically have at least several hundred distinct isotopologues; many such molecules have 106 or more distinct isotopologues. Abundances of only a small subset of these species (typically 2-5) are meaningfully constrained by commonly recognized methods of isotopic analysis.
Although other methods have been developed to expand the set of isotopologues that can be analyzed with useful precision, these methods are applicable to a relatively narrow range of sample types and sizes and to a restricted range of isotopic species in a given analyte target. For example, demonstrated site-specific natural isotope fractionation-nuclear magnetic resonance (SNIF-NMR) techniques can determine, for example, relative deuterium concentration and specific deuterium-site locations in a molecule based on the deuterium NMR signal obtained for the molecule. Comparison of the relative deuterium concentration of the molecule with known global distributions of hydrogen isotope concentrations can provide information regarding the geographic origin of a sample from which the molecule was obtained. SNIF-NMR techniques, however, are not capable of analyzing abundances of molecules containing two or more rare isotopes at their natural abundances and, more generally, require sample sizes that are prohibitively large for many applications and require relatively long, costly analyses. Similarly, established “clumped isotope” mass spectrometric methods can analyze only a few isotopologues of small, simple, highly volatile molecules, principally because of their inability to resolve isobaric interferences and the poor sensitivity of existing gas source multi-collector sector mass spectrometers. Clumped isotope geochemistry and related techniques are described in more detail in “Clumped-isotope' geochemistry—The study of naturally-occurring, multiply-substituted isotopologues,” Earth and Planetary Science Letters, Vol. 262, Issues 3-4, pages 309-327, the entire contents of which are herein incorporated by reference.